A ferrous-alloy article having a surface carbide layer of at least one Va-Group element, such as a vanadium-carbide, a niobium-carbide or a tantalum carbide surface layer, has excellent wear resistance, but poor oxidation resistance. A corresponding alloy article having a chromium carbide surface layer has excellent high-temperature-oxidation resistance and is thus useful, e.g., for a metal mold which is subjected to high temperatures; however, the chromium-carbide layer is inferior in wear resistance to that of a carbide of a Va-Group element.
On surfaces of hot-forging molds, casting-metal molds, glass-forming metal molds, parts of a forging machine, parts of glass-forming machines and the like, which are ordinarily used at high temperatures and are subject to wear-resistance problems, a chromium layer is often formed by a diffusion treatment called chromizing. The most practical surface layer formed on steel is a chromium-carbide layer. As indicated, a chromium-carbide layer has sufficient performance with regard to high-temperature-oxidation resistance but is deficient in wear resistance.
After intensive work a way was devised to impart temperature-oxidation resistance and wear resistance to a ferrous-alloy article.
By forming a mixed-carbide layer of one or more Va-Group elements and of chromium on a ferrous-alloy article surface, the combined merits of both individual carbide layers are imparted to the surface.
To form a mixed-carbide layer on a ferrous-alloy article, immersing and keeping the ferrous-alloy article in a molten borate bath, to which chromium (in the form of pure metal or alloy) powder and powder of one or more Va-Group elements (in the form of pure metal or alloy) are added, is proposed by U.S. Pat. No. 3,671,297. This method is useful but has the following drawbacks:
(1) The metal powders added to the molten borate bath tend to build up at the bottom of the vessel in which the bath is contained. When an article to be treated is immersed in the bath and contacts collected metal powder, some metal powder adheres to the surface of the article, impairing the surface condition of the article. To minimize this, great care must be taken in the dipping of the article to be treated into the bath.
(2) Because of the noted powder buildup, the effective bath volume into which an article to be treated can be introduced is limited, and productivity is continually reduced.
(3) Since particles of the built-up metal powders tend to be sintered with each other in a grain, the effective surface area of the particles is reduced sufficiently to impair and even prevent solution of carbide-forming elements, such as chromium and Va-Group elements, in molten borax. In order to counteract this defect to some extent, it is necessary to stir up the bottom part of the bath-containing vessel from time to time. This necessitates considerable work. (4) When particles of metal powders contact the surface of an article being treated, the particles adhere to and impair the surface condition of the article.
(5) When a large amount of metal powder is added to a salt bath, the viscosity of the bath is increased. The amount of treating material which attaches itself to a treated article and is thus removed from the bath with the article is similarly increased. This reduces bath volume and results in more interruptions for additional bath make-up.
(6) An increase in salt-bath viscosity reduces the fluid character of and the convection flow in the molten treating bath and results in reducing the uniformity in temperature distribution throughout the bath. This further limits the effective volume of the bath-containing vessel.